Asthma is a reversible bronchoconstriction along with mucosal edema. Asthma is a common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction and bronchospasm. Common symptoms include wheezing, coughing, chest tightness, and shortness of breath.
Asthma is thought to be caused by a combination of genetic and environmental factors. Its diagnosis is usually based on the pattern of symptoms, response to therapy over time and spirometry. It is clinically classified according to the frequency of symptoms, forced expiratory volume in one second (FEV1), and peak expiratory flow rate. Asthma may also be classified as atopic (extrinsic) or non-atopic (intrinsic) where atopy refers to a predisposition toward developing type 1 hypersensitivity reactions.
The usual management is to administer inhaled bronchodilators with oxygen at FiO2 (4 to 6 litres). Treatment of acute symptoms is usually with an inhaled short-acting beta-2 agonist (such as salbutamol) and oral corticosteroids. Long-acting beta agonists (LABA) or antileukotriene agents may be used in addition to inhaled corticosteroids if asthma symptoms remain uncontrolled.
Normal cellular function depends upon a continuous supply of oxygen. Inhaled oxygen diffuses across the alveolar-capillary membrane and into the pulmonary capillary blood. The partial pressure for oxygen in the alveoli (approximately 150 mmHg breathing room air at sea level) is greater than in mixed venous blood (40 mmHg) and in the mitochondria (<10 mmHg). This gradient maintains the arterial oxygen tension (PaO2) and is largely the driving force for oxygen delivery to cells.
Oxygen diffuses into the blood where it is predominantly bound to haemoglobin in red blood cells, with a small proportion being dissolved in plasma. The relationship between PaO2 and haemoglobin is described by the curvilinear oxyhaemoglobin dissociation curve. At a PaO2 above 90 mmHg, the curve is nearly flat, and haemoglobin is almost completely saturated. At lower values of PaO2, the curve falls steeply, promoting release of oxygen to the tissues.
Oxygen affinity, which refers to the ability of haemoglobin to bind or release oxygen, is modulated by pH, CO2 (in part independent of pH), 2,3-diphosphoglycerate (DPG), temperature, and foetal haemoglobin. Lower pH, higher CO2, increased temperature, and a decreased proportion of foetal haemoglobin reduce oxygen affinity. These shifts in affinity promote oxygen uptake in the pulmonary capillaries and release into the tissues.
There are a variety of non-invasive ways in which respiratory support can be provided to preterm infants with apnoea or parenchymal lung disease. These include oxygen via a head box or nasal cannula, nasal continuous positive airways pressure (CPAP) and nasal intermittent positive pressure ventilation (NIPPV).
High flow nasal cannulae (HFNC) are small, thin, tapered cannulae used to deliver oxygen or blended oxygen and air at flow rates of >1 L/min. HFNC can be used to provide high concentrations of oxygen and may deliver positive end-expiratory pressure.
Humidified High-Flow (HHF) oxygen/air is a form of respiratory support in preterm infants where their breathing is spontaneous. It is air-oxygen flow (via blender) of 1-6 L/min via the Fisher & Paykel humidifier. HHF is to be commenced at a flow rate of 5 L/min and can be increased to 6 L/min after consultation with registrar/NS-ANP. Potential for asynchrony in breathing may result in the infant becoming tired over long periods; therefore, good assessment of work of breathing is required. Potential for nasal erosion (although less than with HCPAP) remains.
A non-rebreather mask, or NRB, is a device used in medical emergencies that requires oxygen therapy. An NRB requires that the patient can breathe unassisted, but unlike low flow nasal cannula, the NRB allows for the delivery of higher concentrations of oxygen.
Heated Humidified High Flow (HHHF) Therapy is a type of respiratory support is a method of delivering a high minute flow of medical gas to a patient through an interface intended to provide respiratory support; typically a nasal cannula. The applied gas is heated to a set temperature and humidified, targeting ideal body saturation vapor pressure.
Nasal cannula used for medical gas delivery are usually limited to delivery of 1-6 litres of flow per minute. The percent oxygen inhaled by the patient (FiO2), usually ranges roughly from 24% to 35% as the pure oxygen delivered from the cannula is diluted by entrainment of ambient air (21% oxygen). Flow rates for delivery of oxygen using typical nasal cannula are limited because medical oxygen is anhydrous, and when delivered from a pressurized source the gas cools as it expands with the drop to atmospheric pressure. Delivery of cold dry gas is irritating to the respiratory mucosa, can cause drying and bleeding of the nasal mucosa and can increase metabolic demand by cooling the body.
It can be comprehended from the prior art that there are several methods for increasing flow of oxygen using different techniques and devices. However, each arrangement has its own disadvantages.